1. Field of the Invention
The present invention relates to pigments.
2. Description of the Related Art
Diarylide pigments represent an important class of coloring agents used primarily in the manufacture of inks. The most utilized diarylide pigments include, but are not limited to, the following: Pigment Yellow 12, Pigment Yellow 13, Pigment Yellow 14, Pigment Yellow 17 and Pigment Yellow 83.
It is documented in the prior art literature that the addition of fatty primary amines to an aqueous slurry of Pigment Yellow 12, followed by filtration and drying at an elevated temperature, results in a pigment having superior properties (e.g., increased strength, gloss, transparency, and lower rheology) when used to make a solvent-based printing ink, especially a publication gravure type ink (B.P. No. 1,080,835). There are many variations on this technology which have also been patented. Some include the use of diamines to reduce penetration (E.P. No. 57,880), the use of rosin amine (B.P. No. 1,080,116; B.P. No. 1,288,044), the use of heterocyclic amines (B.P. No. 1,334,570), etc. There are also many patents on the process of forming the azomethine additive (aka Schiff's Base or ketimine). Thus, some patents include pre-reaction of the amine and the acetoacetanilide, followed by reaction with tetrazotized DCB (B.P No. 1,334,570; U.S. Pat. No. 4,643,770). Others utilize reaction of the dry Pigment Yellow 12 in a solvent to form the azomethine (U.S. Pat. No. 4,468,255), but most, simply form the additives by drying the presscake containing the amine.
Although the use of the numerous aforementioned amine treatments afford pigments with improved application properties in solvent-based inks (e.g., publication gravure, nitrocellulose packaging, solvent flexo), the use of amine modification for water-based inks has not been exploited, and is within the scope of the present invention. Accordingly, it has been determined that by producing the azomethine compounds of the present invention, diarylide pigment compositions exhibiting significantly improved application properties in both solvent and water-based ink and coating compositions can be obtained. Furthermore, the amine treatments of the prior art possess certain disadvantages, not associated with the azomethine compounds of this invention. For example, there is a marked tendency of the amine treatment to cause a significant green to red shift in the chromophore of diarylide yellow pigments, most notably Pigment Yellow 12. This restricts the use of many of these amine treatments for the production of very green-shade yellows. Another disadvantage is that most amines of the prior art are very odorous and therefore are not usable in certain applications such as packaging ink and are unpleasant to work with in a manufacturing environment.